1. Field of the Invention
This invention relates to monitoring of electrical parameters and, more particularly, apparatus for measuring and displaying a voltage of an electrical system. The invention also relates to a method for measuring and displaying an electrical parameter of an electrical system.
2. Background Information
One common technique for measuring an alternating current (AC) voltage employs a voltage divider network including scaling resistors, which reduce the voltage and feed the same to a buffer amplifier. The output of the buffer amplifier is processed by an analog-to-digital converter (ADC) as controlled by a microcontroller, or by a microcontroller having a built-in ADC. Typically, the microcontroller drives an output display. The scaling resistors must have precise values or, else, must be suitably manually or automatically adjusted until the output display corresponds to the AC input voltage. Although very precise scaling resistors may be employed, such resistors are relatively expensive and do not address tolerances in other components such as, for example, amplifiers.
U.S. Pat. No. 4,933,631 discloses an amplifier circuit including an amplifier and digitally controlled resistors. The circuit is previously tested to determine calibration errors thereof as a function of amplitude and frequency. The calibration errors are stored in read-only memory of a microprocessor. The microprocessor responds to data signals from the memory as well as other data signals indicative of the amplitude and frequency of a monitored signal, in order to set the amplification factor of the circuit.
U.S. Pat. No. 6,084,394 discloses an electronic measuring device, which improves measurement accuracy by providing a correction factor stored in a correction element when the measuring device is first set up. The device includes an A/D converter, and a subsequent digital signal-processing device, such as a microcomputer and/or a digital signal processor. Signal processing is carried out with the aid of programs or program modules. The device also has a correction element. A first correction factor K1, which corresponds to a first stored reference value R1, is saved in a memory of the correction element. In addition, the correction factor K1 may already contain further multiplication factors or other factors for the purpose of (computational) simplification.
U.S. Pat. No. 6,320,525 discloses converting an offset voltage of an operational amplifier to a digital value using a calibration voltage. In turn, that digital value is subtracted from an actual converted voltage value, in order to eliminate the amplified offset voltage. The calibration voltage is equal to a reference voltage minus a voltage drop on two resistors.
State of the art metering devices, such as for use with electric power distribution systems, incorporate a microcomputer, which offers the opportunity for considerable functionality and flexibility in performing metering functions. Such metering apparatus typically monitors RMS and peak values of currents and voltages, power, energy, power factor, watts, VARs, K-factor, and in some cases, harmonic distortion.
Typically, such metering apparatus has a display on which the various parameters are presented, and a user interface through which the user can interact with the apparatus to select operating modes and conditions and request desired information. The microcomputer generates digital representations of the various parameters from analog input signals. These analog input signals, which represent current and voltage in the distribution system, are scaled down by input circuits to suitable input ranges (e.g., 0-20 ma; 0-10 volt) of the A/D converters of the meter.
It is known to calibrate such metering apparatus with a separate computer. A stable reference input to the meter to be calibrated and the output response of the meter are provided to the computer, which calculates a scaling factor. The scaling factor is then downloaded to the microcomputer of the meter. This is done for each of the inputs to the meter.
U.S. Pat. No. 5,706,214 discloses a meter/monitor, which receives an analog input signal as applied to an input resistor. An A/D converter reads the voltage across the resistor. A digital processor appropriately scales the digital signal corresponding to the analog input with two scaling factors: a zero scale factor which applies a dc offset, and a full-scale factor which in effect adjusts the slope of the conversion function. The digital processor reads the digital value of the analog signal generated by the A/D converter. The zero scale factor is subtracted and, then, the result is multiplied by the full-scale factor.
The meter/monitor also employs a sampling routine, which is initiated by a time interrupt to sample input currents, voltages and analog signals. The sample values are employed to calculate RMS values using stored scaling factors. Two scale factors are applied to the analog input signals: a zero scale factor or offset, and a full scale factor or slope.
For calibrating the monitor/meter, a source providing stable values of voltage and current is connected to input ranging circuits, and is also applied to a precision meter, which provides a reference value for the input parameters. For example, when current is selected for calibration, a screen lists all of the phase currents, IA, IB, IC, IN, and IG, the metered value of each of these currents, and the associated scale factor used to calculate the current from the stable value of current supplied by the source. The particular current to be calibrated is selected by soft switches. When one of the currents is selected, a new screen displays the metered values for all of the currents along with the scale factor for the selected current. Soft keys are employed to increment/decrement the digit values of the scale factor. As the scale factors change, the effects are shown by the real-time metered RMS value, which is displayed next to the scale factor. In calibrating the current, the user adjusts the scale factor up or down to make the metered RMS value shown next to the scale factor as close as possible to the reference value generated by the precision meter.
The meter/monitor also generates analog outputs. A digital processor routine appropriately scales the analog outputs. First, the routine normalizes the parameter value to be output as a 0 to a 100% analog signal. If the output signal is a 0-20 ma signal, then the normalized parameter value is multiplied by an analog output scale factor and is written to a D/A converter.
There is room for improvement in methods and apparatus for measuring and displaying an electrical parameter of an electrical system.
These needs and others are satisfied by the present invention, which obtains a scaling factor without requiring additional components. The scaling factor is determined by representing a nominal voltage value with a first digital value; providing a first analog voltage having a first magnitude, which represents the nominal voltage value; and providing a second analog voltage from the first analog voltage, with the second analog voltage having a second magnitude, which is less than the first magnitude of the first analog voltage. The second analog voltage is converted to a second digital value, which is less than the first digital value. The scaling factor is determined by subtracting the second digital value from the first digital value. Then, the scaling factor is stored in a non-volatile memory. Subsequently, analog voltages are determined from the electrical system voltage. Those analog voltages are converted to digital values. The scaling factor is retrieved from the non-volatile memory and is added to each of the digital values to provide a sum, which sum is displayed.
As one aspect of the invention, an apparatus for measuring and displaying an electrical parameter of an electrical system comprises: a non-volatile memory storing a scaling factor; a divider providing a first analog voltage having a first magnitude from the electrical parameter of the electrical system; an analog to digital converter circuit converting the first analog voltage to a first digital value; a processor comprising: a first routine calculating the scaling factor by subtracting a second digital value from a third digital value, the third digital value representing the nominal value of the electrical parameter of the electrical system, the second digital value being converted by the analog to digital converter circuit from a second analog voltage from the divider, the second analog voltage having a second magnitude, the divider receiving a third analog voltage having a third magnitude, which represents the nominal value, the second magnitude being less than the third magnitude, the second digital value being less than the third digital value, an input circuit for the first routine, and a second routine retrieving the scaling factor from the non-volatile memory and adding the scaling factor to the first digital value to provide a sum; and a display displaying the sum.
Preferably, the divider includes a plurality of scaling resistors, which divide the third analog voltage to provide the second analog voltage. The analog to digital converter circuit may include an analog to digital converter having a minimum digital output value and a maximum digital output value, which is greater than the third digital value, which is greater than the second digital value, which is greater than the minimum digital output value.
The analog to digital converter circuit may include a power supply having an output voltage. The electrical parameter may be a voltage. The scaling resistors may form a voltage divider network, which reduces the voltage of the electrical system to the first analog voltage having the first magnitude, in order that the first analog voltage is less than the output voltage of the power supply.
As another aspect of the invention, a method for measuring and displaying an electrical parameter of an electrical system comprises: representing a nominal value of the electrical parameter of the electrical system with a first digital value; providing a first analog voltage having a first magnitude, which represents the nominal value; providing a second analog voltage from the first analog voltage, the second analog voltage having a second magnitude, which is less than the first magnitude of the first analog voltage; converting the second analog voltage to a second digital value, which is less than the first digital value; determining a scaling factor by subtracting the second digital value from the first digital value; storing the scaling factor in a non-volatile memory; providing a third analog voltage from the electrical parameter of the electrical system; converting the third analog voltage to a third digital value; retrieving the scaling factor from the non-volatile memory; adding the scaling factor to the third digital value to provide a sum; and displaying the sum.
Preferably, the method further comprises employing a plurality of scaling resistors, which divide the first analog voltage to provide the second analog voltage; employing an analog to digital converter having a minimum digital output value and a maximum digital output value; and employing the maximum digital output value, which is greater than the first digital value, which is greater than the second digital value, which is greater than the minimum digital output value.